1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount which is suitably used for example as a suspension bushing or an engine mount for a motor vehicle, and which exhibits a desired vibration damping characteristic based on flow of a fluid contained therein.
2. Discussion of the Prior Art
A so-called cylindrical elastic mount is known as one type of a mounting device which is interposed between two members of a vibration system for flexibly connecting the two members in a vibration-damping fashion. This cylindrical elastic mount is constructed with an elastic or rubber body being formed between an inner metal sleeve, and an outer metal sleeve disposed radially outwardly of the inner sleeve with a suitable radial spacing therebetween. The elastic body elastically connects the inner and outer sleeves. The thus constructed cylindrical elastic mount exhibits an intended vibration damping effect, with respect to a vibrational load applied between the inner and outer sleeves in a direction perpendicular to the axis of the mount. The elastic mount of this type is widely used as a suspension bushing or an engine mount for a motor vehicle.
To meet increasing requirements for enhanced vibration damping/isolating characteristics on recent high-grade cars, there has been proposed a so-called fluid-filled cylindrical elastic mount having a plurality of fluid chambers formed between the inner and outer sleeves, and at least one orifice passage connecting these fluid chambers. Upon application of a vibrational load, relative pressure changes occur between the fluid chambers, whereby a fluid is forced to flow between the chambers through the orifice passage[s). This fluid-filled elastic mount exhibits a significantly improved vibration damping effect based on resonance of the fluid mass in the orifice passage (s), as compared with the above elastic mount which relies only on elasticity of the elastic body for damping vibrations.
In the fluid-filled cylindrical elastic mount as described just above, the orifice passages may be formed between the inner sleeve, and a metallic orifice member fitted on the outer circumferential surface of the inner sleeve, as disclosed in JP-A-62-274128. Alternatively, the orifice passages may be formed between the outer sleeve, and a metallic orifice member fitted in the inner circumferential surface of the outer sleeve, as disclosed in JP-A-56-164242 and JP-A-61-270533.
However, these known elastic mounts require an exclusive orifice-defining member or members intended for forming an orifice passage or passages, resulting in an increased number of components of the elastic mount, and rather complicated construction of the mount. Further, there is limitation in space for forming the orifice passages by the orifice-defining member or members.
To solve the above problem, it is proposed to form a through-hole which extends through the elastic body to communicate with the fluid chambers, so as to provide an orifice passage. However, such an orifice passage tends to suffer from variation in its cross sectional area, due to deformation of the elastic body when the mount receives a dynamic vibrational load or a static load. Due to the phenomenon, the elastic mount exhibits an unstable or unreliable vibration damping characteristic, and is therefore unsuitable for practical use.